The present invention relates to copying and duplicating devices and, more particularly, to such devices which are capable of printing a large number of copies per unit time, utilizing an ink jet printing arrangement. A number of non-contact copying devices are shown in the prior art as disclosed, for instance, in U.S. Pat. No. 1,817,098, issued Aug. 4, 1931, to Ranger et al; U.S. Pat. No. 3,604,846, issued Sept. 14, 1971, to Behane et al; and U.S. Pat. No. RE.27,555, issued Jan. 16, 1973, to Loughren. Each of the devices disclosed in these patents supports a copy sheet on a rotating drum, across which a non-contact printer is translated in a direction parallel to the rotational axis of the drum.
U.S. Pat. No. 3,564,120, issued Feb. 16, 1971, to Taylor, discloses a printer in which a plurality of jet drop print heads are scanned in rotary arcs over a print receiving web which is transported past the print heads. Such prior art devices are configured in a manner as to be adapted generally for use in an office copying or duplicating environment, but they are all considered to be too slow for most office copying applications.
A faster copier is shown in U.S. Pat. No. 3,689,693,issued Sept. 5, 1972, to Cahill et al. The Cahill device includes a plurality of print heads, typically eight, which co-operatively print stripes collectively representing an original image which is to be reproduced. This arrangement reduces the printing time of the copies which are made, but the system is somewhat limited by the fact that it has only one printing nozzle per print head. An even faster printing arrangement employs multiple nozzle heads which print interlaced helical tracks, as taught in U.S. Pat. No. 4,009,332, issued Feb. 22, 1977, to Van Hook.
A copier utilizing an ink jet printer generating a plurality of jets which print along interlaced helical print lines on a sheet of copy paper is disclosed in U.S. Application Ser. No. 789,417, filed April 21, 1977, by Paranjpe et al. The Paranjpe et al device contemplates mounting a sheet of copy paper on a paper supporting drum and translating a print head past the rotating drum. An arrangement is disclosed for synchronizing the operation of the print head and rotation of the drum upon which the sheet of copy paper is mounted with optically scanning a master image which is to be reproduced.
A major limitation in the number of copies which may be made per unit time by a copier, having a sheet of copy paper mounted on a rotating drum past which a print head moves, is the time required for loading a sheet of copy paper onto the drum prior to printing and for removing it from the drum after printing is completed. The minimum time for unloading and loading a sheet of copy paper onto a drum is approximately 0.5 seconds, assuming that the drum must be substantially slowed during loading and unloading. Thus, it is clear that even if the time required to print a sheet of copy paper were to be reduced to zero seconds, the copy rate would be limited to 120 copies per minute. As a practical matter, of course, printing an image on a sheet of copy paper will take a finite period of time, and therefore the maximum theoretical copy rate is substantially less than this rate. Where multiple copies of a document are to be printed, it is highly desirable to be able to print at a higher rate.
Another limitation on the maximum copy rate obtainable is the time required to clear the bar. When an interlaced printing arrangement is utilized, such as disclosed in the above-referenced Paranjpe et al application, the print head must be initially positioned such that only a number of the jets at a first end of the head are positioned above the edge of the copy paper which is to be printed first. The print head is then slowly moved past the rotating drum upon which the sheet of copy paper is mounted, with each of the jets depositing drops along interlaced print lines on the sheet of copy paper. The print head must continue to move past the rotating drum until only a number of jets emanating from the second end of the print head are positioned above the sheet of copy paper at the edge of the paper which is to be printed last. The time required for moving a portion of the print head past the edge of the copy paper which is last printed is termed the "time to clear the print head"or bar. It develops that the time to clear the head is approximately constant for a given print resolution, utilizing a bar having a given number of jets per inch. Table 1 below shows the copy rate for different lengths of print bars for printing one sheet of copy paper at a time mounted on a rotating drum.
Table 1 ______________________________________ Time Time to to No. Bar Clear, Load/ Total of Width Time to Bar, Unload Time Copies/ Jets Inches Print Sec. Sec. Sec. Sec. Min. ______________________________________ 1 233.75 .5 234.25 .26 20 .2 11.7 .275 .5 12.475 4.8 100 1.0 2.34 .275 .5 3.11 19.2 500 5 0.468 .275 .5 1.243 48 1000 10 0.234 .275 .5 1.01 59 3000 30 0.078 .275 .5 0.853 70 ______________________________________
The above table was computed on the basis of 100 KHz stimulation frequency and 0.002 inch dot-to-dot spacing to cover an 8 1/2inch by 11 inch sheet of copy paper. It is seen from the above table that the time required to load and unload the paper and the time required to clear the print head become increasingly significant factors in the copy rate as the rate increases. For example, at 59 copies per minute, the time required for actual printing of a document is only 23% of the total time required to produce the copy. The time required for actual printing of a document at 70 copies per minute is only 9% of the total time required to produce the copy.
Thus, it is seen that a need exists for an improved ink jet copier for printing multiple copies in which the copy rate is increased.